This invention relates to document handling systems in general and more specifically to paper sensors for document handling systems having automatic paper feeders.
Document handling systems with automatic paper feeders are found in a wide range of devices, such as photocopy machines, facsimile machines, document scanners, and printers for computers, just to name a few. Most document handling systems in use today utilize one or more paper sensors to sense the location of paper as it moves through the various paper paths within the machine. Such paper sensors allow the control system of the machine to monitor the progress of the paper as it passes through the paper path, as well as to synchronize and coordinate the various paper handling functions.
A typical paper sensor comprises a moveable sensing vane connected to a switch. The switch may be either mechanical or optical and will be in one state when paper is present in the path and the opposite state when there is no paper in the path. Paper moving through the paper path displaces the sensing vane, which causes the switch to change back and forth between states. By sensing the current state of the switch, as well as the length of time that the switch remains in that particular state, the control system can determine whether paper is present at that location, as well as how long the paper has been at that location. Such information allows the control system to detect no-feeds and jams.
For example, if paper is not present in the path at the correct time during a paper feed cycle, then a no-feed or jam has occurred. Similarly, jams that occur while the paper is present at the sensor location may be detected by monitoring the length of time that the paper is sensed. If the time is longer than the time the paper should be at that station, then a jam has occurred.
While paper sensors of the type described above are useful in detecting paper jams and no-feeds, they cannot detect a multiple page feed, that is, a feed that occurs when two or more sheets of paper are fed through the paper path at the same time. Of course, if a multiple page feed occurs with original documents being copied, such as, for example, by a copy machine, then the second sheet will not be copied. Multiple page feeds also pose a problem in automatic document feeders for scanners used to scan paper sheets into an electronic database, since the operator must constantly monitor the scanner to ensure that no pages are skipped.
Consequently, there remains a need for a paper sensor for document handling systems that can not only detect paper jams and no-feeds, but that can detect multiple page feeds as well.